The AR-15 assault rifle has been in production since 1959, first by ArmaLite, then Colt, and in the preceding years by any number of firearms manufacturers. It was used as the basis for the iconic M-16, and a multitude of other semi-automatic, and automatic rifles. In the intervening years it has become near ubiquitous, and as such the many variants of the basic AR-15 platform come in a variety of sizes, calibers, and barrel lengths. The varieties include not less than 8 different barrel lengths, ranging from 24 inches down to 6.5 inches, and over 65 calibers can be fired using the basic AR-15 platform. In addition to those choices, an AR-15 owner literally has thousands of aftermarket add-ons from which to choose including hand grips, sights, scopes, lasers, lights, and rails for attaching this multitude of accessories. As a result, the AR-15 is perhaps the most versatile and customizable firearm platform on the market today.
In light of this, the present invention is an adapter which replaces the barrel nut assembly on an AR-15 platform. This adapter allows the user to quickly change barrel systems without special tools. The ability to change barrels is the key to modularity for the AR-15 platform; as the barrel holds the chamber and the chamber determines the caliber.